On April 17th, Rei is walking down the street when a boy comes up to her and gives her a bouquet of flowers for her birthday. Usagi sees this and snatches the letter away from Rei and reads it, while Ami observes that they never knew it was Rei's birthday. Rei, however, is completely uninterested, and shoves the flowers into Usagi's hands, declaring that she has no interest in presents or boys, before she walks away. Ami and Usagi watch her go, noting that they don't know much about Rei since she goes to a different school and doesn't talk about herself very much.

Rei arrives at the Hikawa Shrine and is greeted by her grandfather, who tells her that a present has arrived from her father. When Rei opens the present, she finds a white dress and a bouquet of flowers, and thinks that it's the same present again this year. Usagi and Ami arrive and give Rei a music box as a birthday present, and she thanks them while looking somewhat embarassed. When Usagi admires the flowers, Rei says that they're Casablanca lilies, her favorite flowers. A car arrives to pick Rei up, and she leaves. Her grandfather tells the two girls that every year on her birthday, Rei has dinner with her father, a busy politician who she hasn't gotten along with since her mother died.

Rei arrives at the Rain Tree restaurant, and when she arrives at her table finds her father's secretary, Kaidou, also there with her father. They make brief small talk, and Rei notices a music box nearby that looks like the one Ami and Usagi gave her. The waiter notices her interest and explains that the song the music box plays is the same as the name of the restaurant, "Rain Tree". Rei's father asks if she came to the restaurant the previous year with Kaidou, and Rei says that she did, since her father couldn't make it.

Rei thinks about Kaidou and how she's known him for years; he always took care of her, just like a brother. She remembers the time she and Kaidou walked by a flower shop and she admired a display of Casablanca lilies, only to receive some of those flowers along with a white dress for her birthday later. That was when she realized that her birthday gifts were chosen by Kaidou and not her father. The previous year, she recalls, when she spent her birthday with Kaidou, she declared that she couldn't have faith in anyone and in the end realized she could only depend on herself. Because of this, she said, she never wanted to marry. Kaidou agreed with her then, and she said that the two of them were kindred spirits.

Rei sits up in bed, shaking her head to clear it of past memories. She glances at the music box nearby, then goes to meditate before her fire, but has a hard time concentrating.

That June, two of Rei's classmates look out the window at the constant rain and observe how gloomy it is. One of them spots a Rain Tree music box and delightedly exclaims that she has one, too.

Usagi stares out her bedroom window and declares that the start of the rainy season is depressing. Ami works on homework at the table behind her, noting that this time of year also brings a lot of homework. She adds that the popular Rain Tree music boxes seem to have the perfect melody for the season. Usagi's mother agrees, saying the music makes you remember the past. Luna peeks over at the others and wonders if the rain is what's making everyone so glum lately.

Rei walks down the street with an umbrella, and runs into Makoto outside of a flower shop. Makoto explains that the rain has been hard on her plants at home, and she went to get some fertilizer for them. She looks over at people passing by and says that everyone seems to be so gloomy, and adds that she's been hearing that sad Rain Tree melody often lately. Rei agrees, and suggests that the two of them go somewhere to talk.

The two girls enter the Rain Tree restaurant, and Rei is greeted by the waiter as Makoto looks uncomfortable. After they're seated, Rei mentions that Makoto said something about her heart being broken in the past. Makoto replies that she and a friend both liked the same person, but he didn't choose her. Rei replies that they're kindred spirits, but Makoto only looks confused.

Rei remembers saying the same thing to Kaidou, but shortly afterwards seeing him acting affectionate with an unfamiliar woman. Feeling betrayed, she ran away, and Kaidou saw her go. She called her father, who told her that Kaidou was going to succeed him someday; this girl was the daughter of the head of the Democratic Liberal Party, making the two of them a perfect couple.

She awakes from her reverie and goes over to the grand piano, playing the theme of the movie Casablanca. After she finishes, another woman sits down to play, and Rei seems to sense something odd about her. She doesn't stop to think about it, though, as she suddenly feels like there's something wrong with her that's making her feel so oddly sentimental. Rei returns to Makoto, who has fallen asleep, and the two of them leave. Feeling much relieved once in the fresh air, Rei wonders why the air was so oppressive inside.

At the Tsukino house, Luna reads in the paper that the strangely heavy rains are causing floods, and Usagi adds that so many students were absent because of the rain that they had to close the school.

Rei walks through the rain carrying a vase of flowers, bringing them to Makoto. As she passes a display of Rain Tree music boxes, she thinks that she's starting to get irritated with that almost constant melody. When she arrives at Makoto's apartment there is no answer, but the door is unlocked so she enters. Once inside, she finds Makoto hunched over in a corner, staring forlornly at a Rain Tree music box and mumbling about her senpai. Realizing something is wrong, Rei picks up the music box and hurls it to the ground, shattering it, which wakes Makoto from her stupor. However, an evil spirit starts to emerge from within the broken music box. Rei realizes that an enemy was saturating the city with the possessed music boxes and transforms into Sailor Mars.

As she does, memory returns of a time when Rei was at the shrine and Kaidou came to give her a book about lilies, claiming that her father asked him to give it to her. Rei grew annoyed with him and asked why he always treated her like a child, pretending that everything was from her father, then demands to know if he was getting married. Kaidou replied that he had to, because her father said to. She remembered thinking they were kindred spirits and accused him of being a liar, then added that she wanted him to marry her - and he leaned over to kiss her.

In the present, Sailor Mars cries out and the Rain Trees all shatter. She hears music, and the woman who was playing the piano at the Rain Tree restaurant appears, along with a giant translucent Rain Tree. The woman declares that she used the music boxes and the flooding rain to collect the sentimental energy of humans, and she intends to soak up all the energy in the city, leaving the people nothing but empty shells.

Old memories flood into Sailor Mars, and she remembers Kaidou, and her mother, and then Jadeite's face appears, merging with Kaidou's in her mind and forming an evil-looking apparition. Sailor Mars angrily declares that she will not dwell on the past, and calls upon Phobos and Deimos to aid her. She uses Akuryo Taisan and blasts the woman, who is revealed to be Zoisite in disguise; he was trying to get revenge for Jadeite's death, but found she was too strong.

That August, Rei lounges on a beach chair in the sun. Makoto squints up at the bright sun and declares that the dismal rainy season now seems like it was just a bad dream. Usagi calls over to invite Rei to come swimming, but Rei insists she's fine where she is. A waiter arrives to bring her a drink sent by a couple of nearby boys, and Rei smirks and says that's her fifth one; she's perfectly happy to sit there and drink all day long. Her friends look on, expressions varying between jealousy and disgust.

The title of this story is a reference to a type of oriental lilies known as "Casablanca," which is Michie Tomizawa's favorite flower.

When Sailor Mars broke free of Zoisite's power and shattered the music boxes, she cried out "katsu," a shout used in Buddhism and martial arts. In the French manga, however, she shouted "Grondement de Mars" (Mars Growl), as though she was performing an attack.

There was a reference to this side story in the live-action episode Act 8 - Rei and Her Father. In that episode, Rei and her father were supposed to have their yearly dinner together, and he gave her a dress and a bouquet of flowers for her birthday.